Consider the person's posture , gestures , and stance. Generally people who are attracted to each other try to close the distance to one another. This may mean leaning forward toward the other person more but can also be more direct , in the form of touching. A light tap or stroking of the arm can be a signal of attraction.
Attraction can also be signaled by the person's feet staying pointed toward or facing the object of his interest.
Palms turned up is another sign of romantic interest because it suggests openness.